This invention relates to a printing apparatus, such as a copying machine, a printer and a facsimile, in which the viscosity of a toner image is reduced to a low valve by use of a solvent of said toner before transfer of the toner image for obtaining a hard copy.
Typical examples of conventional recording systems utilizing toners include electrophotography and electrostatic recording. In such systems, an electrostatic latent image is formed on a charge carrier such as an photoconductive medium and an electrostatic recording medium, and this latent image is developed by charged toner having a polarity opposite to that of the latent image, so as to obtain a toner image. Then, the toner image thus obtained is fixed into a final recorded form.
In the electrostatic recording, the electrostatic latent image is formed on electrostatic recording paper by a recording electrode or the like, and this latent image is developed by the toner. This electrostatic recording paper serves as the charge carrier and an image bearing medium. Therefore, the electrostatic recording system obviates a step of transferring the toner image from the charge carrier to the image bearing medium. On the contrary, the electrophotography inevitably requires the step of transferring the toner image from the photosensitive medium (charge carrier) to the image bearing medium, and the step of fixing the thus transferred image.
The present invention is devised so as to improve the step of transferring a toner image, for example, in the electrophotography, or the step of fixing a transferred image.
Examples of known transfer systems of transferring a toner image from a charge carrier to an image bearing medium include an electrostatic transfer method, an adhesion transfer method and a pressure transfer method.
First, the electrostatic transfer method will now be described. Examples of such electrostatic transfer methods include a corona transfer method and a bias transfer method. In the corona transfer method, an image bearing paper sheet is set on the charge carrier, and a charge of a polarity opposite to that of the toner charge is applied by a corona charger to the paper of the reverse side of the charge carrier, thereby transferring the toner image to the paper sheet under Coulomb force. On the contrary in the bias transfer method, an image bearing paper sheet is set on a toner image on a photoconductive drum, and a conductive electrode drum to which direct current is applied is pressed against the reverse side of the paper sheet so that the toner image is transferred onto the paper sheet by Coulomb force.
However, when using the corona transfer method so as to transfer the toner image, there has been encountered a problem such that the toner transfer efficiency greatly depends on the electric resistance of the image bearing sheet. Particularly in a high humidity condition, the image bearing sheet absorbs the moisture so that its electric resistance decreases, and as a result, the toner image could not be transferred to the image bearing sheet at all. This offers a serious problem. Furthermore, during the corona transfer, the toner image is susceptible to scattering, resulting in a problem that the printed image is dim.
Further, in an electrophotographic apparatus employing the corona transfer, high resistance toner is used. It is said that the critical resolution, when using the high resistance toner, is equal to four to five times as large as than the particle size of the toner particles. Therefore, a printed image of high resolution could not be expected.
In an ordinary two-component development using high resistance toner, there has been encountered a problem of a so-called edge effect.
In order to overcome the degrading of the image quality caused by the use of the high resistance toner, it has been proposed to use electrically-conductive toner. Such a method is disclosed, for example, in U.S. Pat. No. 3,639,245.
However, when the conductive toner is used, the transfer can not be carried out by the corona transfer method. Therefore, when the conductive toner is used, adhesion transfer method using an intermediate medium, the above-mentioned pressure transfer method, or other methods should be employed.
Next, the adhesion transfer method will now be described. In the adhesion transfer method, the toner is transferred utilizing the adherent or sticky force of a image bearing sheet. Althernatively, when transferring the toner to ordinary paper, the toner image is once transferred to an intermediate transfer medium, and then is again transferred therefrom to the ordinary paper.
Such an adhesion transfer method is described, for example, in Japanese Patent Eexamined Publication No. 46-41679. However, at the steps of retransferring the toner image from the intermediate transfer medium to the image bearing sheet and fixing the thus transferred image, the toner on the intermediate transfer medium is heated by a heating roller so as to be rendered viscous. Therefore, the intermediate transfer member is heated by the heating roller, and the photoconductive member in contact with the intermediate transfer medium must be also heated. As a result, the photoconductive medium undergoes thermal effects, which causes adverse effects such as a variation in the charge characteristics of the photoconductive medium and a shortened service life of the photoconductive medium.
Next the pressure transfer method will now be described. In the pressure transfer method, pressure-fixing toner is used, and an image bearing paper sheet is held between a photoconductive drum and a press roller. The toner is transferred and fixed to the image bearing sheet by a pressure, thus simultaneously effecting the transfer and the fixing.
In this method, since a high pressure is applied to the appartus, the apparatus is required to have an increased mechanical strength, and the drive torque is also increased. Therefore, there has been encountered a problem that only the type of photosensitive members having a high pressure resistance can be used. Furthermore, the image bearing paper sheet is subjected to calendering because a high pressure is applied to the image bearing sheet. For this reason, a gloss develops on the surface of the image bearing sheet, and the fixing ability is inferior to that of the other fixing methods.
(b) Examples of conventional fixing methods include a thermal fixing method, a pressure fixing method and a solvent vapor fixing method.
First, the thermal fixing method will now be described. In the thermal fixing method, the fixing is carried out by melting toner by means of a heating roller. This method is widely used at present. In this method, when electric power is supplied to the apparatus to initiate its operation, there is required a certain period of time for warming up the apparatus. Also, a large amount of thermal energy is required for effecting the fixing. For these reasons, a power source of a large capacity is needed for the heating roller, which results in a problem that the electric power consumption of the apparatus is large. Furthermore, the fixing is carried out by heat, and therefore even if it is desired to increase the fixing rate, this can not be achieved because the temperature of the heating roller can not be increased from the viewpoint of safety. Thus, this method suffers from such a drawback that the recording can not be carried out at a high speed.
Next, the pressure fixing method will now be described. Toner used in this pressure fixing method is made of a pressure sensitive material, the viscosity of which is lowered when pressure is applied to the toner. Examples of such toner materials include paraffin, wax and rubber-like soft polymer. In this method, a high pressure is applied to the toner to lower its viscosity, so that the toner can penetrate into the fibers of the image bearing paper sheet, thereby carrying out the transfer and fixing. This method has offered problems such that the calendering of the image bearing sheet produces a gloss on the printed image and lowers the fixing ability as described above, that the weight balance of the copying machine is not good due to a heavy weight, resulting in less portability, and that impact noise is produced when the paper sheet is discharged from the copying machine.
Next, the solvent vapor fixing method will now be described. In this solvent vapor fixing method, solvent vapor dissolves a toner image on an image bearing paper sheet, thereby fixing the toner image to the image bearing sheet. When it is considered that this method is used in various environments, several problems are encountered. Namely, it is difficult to control the pressure of the solvent vapor to a constant level. It is necesasry to provide means for preventing the leakage of the solvent vapor. It is difficult to use cut paper sheets.
The conventional printing apparatuses which have been widely available employ a combination of the aforesaid electrostatic transfer (the corona transfer method in particular) and the thermal fixing.
(c) Japanese Patent Unexamined Publication No. 62-67576 discloses a prior art method in which a toner image formed on an image carrier is dissolved by a solvent so as to make the toner image viscous, and the viscous toner image is transferred to an image bearing medium through adhesion. This method will now be described.
First, a toner image is formed on a photoconductive drum by an ordinary electrophotographic process. Then, a toner powder-dissolving drum (hereinafter referred to as "dissolving drum") coated with a solvent capable of dissolving the toner is in opposite to the photoconductive drum with a space there between. Then, the toner image on the photoconductive drum is electrostatically transferred to the dissolving drum by a transfer charging device mounted within the dissolving drum. The toner thus transferred to the dissolving drum is dissolved by the solvent and is made to be viscous. In this condition, when a paper sheet is brought into intimate contact with the toner image, the toner image is adhesion-transferred to the paper sheet by the adherent or sticky force of the viscous toner.
However, the printing apparatus of this type has the following problems:
(1) A sharp printed image can not be obtained.
The reason for this will be described. If there is a gap between the photoconductive drum and the dissolving drum when the toner image on the photosensitive drum is to be electrostatically transferred to the dissolving drum, the toner image is scattered because of a scattering phenomenon as is well known in the art. As a result, the transferred toner image on the dissolving drum is dim. Thus, the above-mentioned problems of the conventional electrostatic transfer method have not been solved.
(2) To deal with the above scattering problem, it is considered to effect the electrostatic transfer by making the photoconductive drum into intimate contact with the dissolving drum. However, with the conventional method, a new problem arises such that the resultant printed image has increased void portions and therefore is unclear.
The reason for this is that even if a sharp toner image is transferred to the dissolving drum, the toner image made to be viscous by the solvent is adhesion-transferred while being distributed possively to both photoconductive drum and dissolving drum.
(3) The solvent begins the dissolve of the toner transferred to the dissolving drum from that surface of the toner making contact with the dissolving drum toward the paper sheet to which the toner is to be transferred. In other words, when viewed from the side of the paper sheet, the toner is dissolved from its inner surface toward its outer surface.
Accordingly the toner is made to be viscous earlier at its inner side adjacent to the dissolving drum than at its outer side adjacent to the paper sheet. For this reason, if the solvent supplied to the dissolving drum is in a small amount, the adherent force acts only on the dissolving drum side and does not act on the paper sheet side, and as a result the toner is not transferred to the paper sheet at all, thus failing to obtain a printed image. On the other hand, if the solvent is suppted in an increased amount, the dissolved toner is difused, and therefore the lines of the printed image become thick, thus affecting the reproduction of thin lines.
Anyway, in such a method in which the solvent is applied to the toner from the inner surface of the toner, it is extremely difficult to control the amount of the solvent, and therefore it is difficult to stably obtain a clear printed image.